Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already cover safety (readOnly, non-destructive), but the description adds substantial context they cannot: the price and payment rails, the error shape on unpaid calls, the fresh invoiceId per call (which explains idempotentHint=false), and the stated accuracy bounds (~1 min within 72 degrees latitude, up to 10 min beyond). The provenance disclosure (NOAA/GML equations after Meeus, not NOAA-endorsed) is exactly the kind of behavior an agent needs. It stops short of describing output field structure, but no output schema exists and UTC instants are already named.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with the cost and rails, then the question answered, then provenance and accuracy, then the payment failure path — a sensible order for a paid tool. Minor waste: 'Sunrise, solar noon and sunset' is restated in consecutive sentences, and the payment rail information is duplicated between title and description.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a paid, open-world calculation tool with no output schema, the description covers cost, rails, retry mechanics, error behavior, computation source, and accuracy limits — everything an agent needs to call it correctly and interpret the result. Nothing material is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so lat, lon, date, and payment_signature are already documented, including range and date format. The description only reinforces the payment_signature fallback path and adds no syntax or interpretation detail beyond the schema. Baseline 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

Names a specific resource (sunrise, solar noon, sunset) and the exact scope (this point, this date), and explicitly frames the output as UTC instants. The line 'A calculation, not an observation; not NOAA data' separates it from observation-style siblings such as get_current_conditions without requiring the agent to inspect another schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

States the single question the tool answers and gives the invocation protocol: first call without payment returns PaymentRequired, then retry with the signed payload in _meta["x402/payment"] or payment_signature. It does not name a non-paid alternative for solar times, so the when-not guidance is only implicit.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.